1. Field of the Invention
The present invention relates to data transmission systems for communications and, in more particular, to a large-capacity data transmission architecture for time-division/optical frequency-division multiplexed information signals using coherent optical data transmission lines.
2. Description
With the increasing needs for high performance and reliability of data communications systems, development of optical high-density data transmission technology with a large capacity becomes indispensable which can send out and take in highly multiplexed opto-electrical information signals. Attention should be paid to the fact that in recent years an advanced broadband digital information communications network system has been developed which can provide, in addition to voice service, several kinds of services handling high-speed data and video image data that are transmitted simultaneously among terminal stations or local stations. Such type of network system has been known as the "broadband integrated service of digital network," or for short, the "B-ISDN."
In the "B-ISDN" system, information signals will be typically time-division multiplexed with signal transmission rate ranging 150 and 160 mega-bit per second (Mbps) 155.52 Mbps, for example as a unit. It will not be expected that such high-density signals are processed by using the conventional large-capacity data transmission technology using the old combination of semiconductor laser modulation scheme and direct demodulation. The reason of for this is as follows: even if the presently available "large-capacity" data transmission technology offering the maximum rate of 10 giga-bit per second (Gbps), data communication lines among terminal stations cannot provides more than 64 line-channels. Accordingly, it has been strongly demanded in the art to develop a powerful data transmission technique that can break the "64 lines" limitation, and can offer an extra large capacity of data transmission, which will be at least several tens times as great as that of the currently available technology, or preferably, much greater than that.
With the highly advanced extra large-capacity data transmission system, a digital data exchange unit, known as the "digital switching unit," which is provided within each of the terminal stations for data exchange among other terminal stations associated therewith, will naturally be complicated in its internal arrangement. In order to internally exchange all of the high-speed and high-density data signals being supplied from the increased number of terminal stations, the digital switching unit should be required to "decode" the individual signal of multiplexed input data signals externally supplied thereto, branch-exchange the decoded signals, and "encode" again for the purpose of external transmission. Accordingly, if the conventional digital switching technology is simply applied to such extra large capacity data transmission system as it is, the data processing load of every internal switching unit will become heavier incredibly. This makes it impossible to exhibit the full advantage of extra large capacity, high speed, and high quality data transmission that is inherently executable in the highly advanced data transmission system.
Some of the serious problems in internal switching exchange units of the aforementioned data transmission system of "extra-large" capacity will be explained hereinafter in more practical manner. It is to be assumed in a B-ISDN system that a signal transmission unit is set to 600 Mbps for the aim of simplicity of explanation. A high-definition TV image signal may be compressed by using a conventional band compression technology to a signal level of 150 Mbps, so that 600 Mbps corresponds to the amount of four channels of normal high-definition TV image signals. It should be noted that the discussion presented below will also be true even if the signal unit is changed to a different value, such as about 50 Mbps, or 150 Mbps; it will be true even when it is applied to a different type of extra-large capacity data transmission system other than the B-ISDN type system.
In the B-ISDN type system, data transmission in an a synchronous transfer mode, known as "ATM," will be utilized with higher possibility for at least base communication lines of the system in order to maximize the flexibility of signals to be processed and at the same time the efficiency in use of data transmission band path. With such ATM transmission scheme, each of data signals to be transmitted is time-division multiplexed with a "cell" which consists of a header section and a data section being as a unit. In this case, a coherent optical frequency-division multiplexing (FDM) transmission system will be preferably used, wherein a hundred of signal channels can be processed at a time and each of the channels may be time-division multiplexed at a level of 2.5 Gbps, which is about sixteen times that of the four channel-compressed high-definition TV image signal.
Optical fiber transmission paths, or optical links, briefly, are provided among terminal stations in such a manner that each of these terminal stations is connected with a plurality of optical links to enable it to communicate with some of these terminals simultaneously. Typically, each terminal station may be supplied with input signals of several thousands of channels, or more, each of which provides unit signals of 600 Mbps. In that case, an internal data switching unit provided inside each of the terminal stations may be a 800.times.800 ATM switching network which has four stages of 256 module arrays: each module includes large-scale integrated (LSI) ATM switches of 32 rows and 32 columns. These self-routing switches are such switches the outputs of which may be automatically determined depending upon the contents of the header section of the signal. The fact that the transmission rate of the input signals is as high as 600 Mbps should require the self-routing switches to operate at enhanced switching rate enabling to deal with such high-speed input signals. It should also be required that internal electrical signal lines provided inside the data switching unit for transmitting the high-speed data signals at 600 Mbps are short in length as much as possible and adjusted to exhibit the same transmission delay thereamong.
Taking the above conditions into consideration, it is highly required for the individual internal digital switching unit, which is provided in each terminal station under the B-ISDN system, to be made simple in circuit configuration containing complex electrical signal transmission lines. With a presently available digital data switching technology, however, the internal switching units are very difficult or unable to meet such technical requirement. The reason for this is as follows: the data transmission line path, naturally, becomes more complex as the optical links associated therewith are increased in number each of which allows to deal with high-speed data signals at increased transmission rate.